Merge changes from topic "st-nand-backup-fwu" into integration

* changes:
  refactor(st): rename plat_set_image_source
  feat(st): add FWU with boot from NAND
  feat(st): manage backup partitions for NAND devices
  feat(bl): add plat handler for image loading
  refactor(bl)!: remove unused plat_try_next_boot_source